|
7 | 7 | * |
8 | 8 | * Copyright (c) 1994, Regents of the University of California |
9 | 9 | * |
10 | | - * $Id: pg_attribute.h,v 1.48 1999/07/31 19:07:25 tgl Exp $ |
| 10 | + * $Id: pg_attribute.h,v 1.49 1999/08/09 02:45:56 tgl Exp $ |
11 | 11 | * |
12 | 12 | * NOTES |
13 | 13 | * the genbki.sh script reads this file and generates .bki |
@@ -53,8 +53,15 @@ CATALOG(pg_attribute) BOOTSTRAP |
53 | 53 |
|
54 | 54 | float4attdisbursion; |
55 | 55 | /* |
56 | | - * attdisbursion is the disbursion statistic of the column, or zero if |
57 | | - * the statistic has not been calculated. |
| 56 | + * attdisbursion is the disbursion statistic of the column (0.0 to 1.0), |
| 57 | + * or zero if the statistic has not been calculated, or -1.0 |
| 58 | + * if VACUUM found that the column contains no duplicate entries |
| 59 | + * (in which case the disbursion should be taken as 1.0/numberOfRows |
| 60 | + * for the current table size). The -1.0 hack is useful because the |
| 61 | + * number of rows may be updated more often than attdisbursion is. |
| 62 | + * We assume that the column will retain its no-duplicate-entry |
| 63 | + * property. (Perhaps this should be driven off the existence of a |
| 64 | + * UNIQUE index for the column, instead of being a statistical guess?) |
58 | 65 | */ |
59 | 66 |
|
60 | 67 | int2attlen; |
|